The processes for converting hydrocarbons at a high temperature, such as, for example, steam-cracking or alternatively, catalytic cracking, provide unsaturated hydrocarbons such as, for example, ethylene, acetylene, propylene, butadiene, butenes, lighter compounds such as methane, hydrogen and carbon monoxide, and hydrocarbons boiling in the gasoline range. Thus, the gaseous monoolefinic hydrocarbons with two or more carbon atoms, obtained by these processes also contain an amount of hydrocarbons of greater unsaturation degree. The content of these hydrocarbons depends on the severity of the conversion treatment, but is often too low to economically justify their separation and their use as such in the petrochemical field. This is often the case of the ethylene and propylene cuts from which acetylenes must be removed.
To this end, the prior art is replete with a number of patents describing catalyst compositions for the selective hydrogenation of these compounds. Exemplary of these prior art disclosures are La Hue et al., U.S. Pat. No. 3,679,762; Cosyns et al., U.S. Pat. No. 4,571,442; Cosyns et al., U.S. Pat. No. 4,347,392; Montgomery, U.S. Pat. No. 4,128,595; Cosyns et al., U.S. Pat. No. 5,059,732 and Liu et al., U.S. Pat. No. 4,762,956.
However, in the olefins plants of the prior art, the acetylene hydrogenation reactor has been located at the back end of the plant. Accordingly, the prior art processes, such as the Acetex process of the Institut Francais du Petrol, require an external fluid solvent and must carefully regulate the hydrogen ratio, carbon monoxide content and reactor inlet temperature due to pressure sensitivities to excursions in acetylene and carbon monoxide concentrations.
Further, prior designs utilizing the acetylene hydrogenation reactor in the front end of the olefins plant have experienced operating upsets due to temperature excursions experienced during initial start up resulting from the sensitivity and activity of the fresh catalyst. Typically, front end units require multiple beds to reduce temperature rise and thus the chances of upsets.
To this end there exists in the art a need for a method to hydrogenate the acetylenes in she front end of an olefin plant process stream and overcome the difficulties of the prior art processes, as discussed above.